FAA Private Pilot Airplane (PAR)Weather Theory and ServicesMedium
While flying over a lake on a cool autumn morning, a pilot notices wisps of fog rising directly off the water's surface as cold air moves across it. This phenomenon is best classified as which type of fog?
- AUpslope fog
- BSteam (evaporation) fog
- CRadiation fog
- DAdvection fog
Show answer & explanationAnswer & explanation
Correct answer: B. Steam (evaporation) fog
Steam fog, also called evaporation fog, forms when cold, dry air moves over relatively warm water. Water evaporates into the cold air and quickly condenses, creating wisps of fog that resemble rising smoke.
Why the other options are wrong
- A. Upslope fog forms when moist air is forced up rising terrain, not applicable over a lake.
- C. Radiation fog forms over land on clear, calm nights due to ground cooling, not over water.
- D. Advection fog forms when warm, moist air moves over a colder surface, the opposite scenario here.
Steam (Evaporation) Fog
Fog that forms when cold air moves over much warmer water, causing evaporation that quickly condenses into visible wisps resembling steam.
- Common in fall over lakes and oceans as water retains summer warmth
- Can cause low-level turbulence and icing due to instability
- Appears as 'sea smoke' rising off the water
Memory trick: Steam fog looks like a hot cup of coffee steaming on a cold day